1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus, which reduces and purifies nitrogen oxide (NOx) in exhaust gas with the use of a liquid reducing agent or its precursor, and a liquid level measuring device which measures the liquid level of the liquid reducing agent or its precursor stored in a reducing agent tank of the exhaust emission purifying apparatus.
2. Description of Related Art
As a catalytic purifying system that removes NOx contained in exhaust gas of an engine, there has been proposed an exhaust emission purifying apparatus described in Japanese Laid-open (Kokai) Patent Application Publication No. 2000-27627. In the exhaust emission purifying apparatus, a liquid reducing agent or its precursor is injection-supplied to the exhaust upstream side of a NOx reduction catalytic converter at the dosing flow rate corresponding to an engine operation state to subject a reducing agent and NOx to reduction reaction selectively in the NOx reduction catalytic converter, and thus, to purify NOx to harmless water (H2O) and nitrogen (N2).
In the above-mentioned exhaust emission purifying apparatus, if the liquid reducing agent or its precursor is insufficient in amount, or if the concentration of the liquid reducing agent or its precursor is inappropriate, the reducing agent supplied to the NOx reduction catalytic converter is insufficient in amount, and a required NOx purification performance may not be achieved. Thus, as described in Japanese Laid-open (Kokai) Patent Application Publication No. 2008-95601, the present applicant has proposed a technique of attaching a sensor in which a liquid level indicator and a concentration meter are unified to a top wall of a reducing agent tank storing the liquid reducing agent or its precursor therein.
In a sensor disclosed in the related art, a concentration meter is attached to a tip end portion of a liquid level indicator that measures the liquid level from a capacitance change between an inner electrode and an outer electrode having an annular-shaped cross section. Therefore, the capacitance between the inner electrode and the outer electrode near the concentration meter changes, so that the liquid level at this portion cannot be accurately measured. In heavy vehicles such as heavy trucks, since the height of the reducing agent tank is large, the ratio of the range in which the liquid level may not be accurately measured is small, and the range can be ignored.